|Publication number||US8004251 B2|
|Application number||US 12/494,026|
|Publication date||Aug 23, 2011|
|Filing date||Jun 29, 2009|
|Priority date||Apr 26, 2007|
|Also published as||CN101688890A, US7570028, US20080270048, US20090278598, WO2008134344A1|
|Publication number||12494026, 494026, US 8004251 B2, US 8004251B2, US-B2-8004251, US8004251 B2, US8004251B2|
|Inventors||Gideon van Zyl|
|Original Assignee||Advances Energy Industries, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (39), Non-Patent Citations (1), Referenced by (9), Classifications (7), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a continuation of U.S. application Ser. No. 11/740,710, entitled: METHOD AND APPARATUS FOR MODIFYING INTERACTIONS BETWEEN AN ELECTRICAL GENERATOR AND A NONLINEAR LOAD, filed Apr. 26, 2007 now U.S. Pat. No. 7,570,028, which is incorporated herein by reference.
The present invention relates generally to electrical generators. In particular, but not by way of limitation, the present invention relates to methods and apparatuses for modifying interactions between an electrical generator and a nonlinear load.
In some applications, it is advantageous to use an electrical generator with a source impedance that is very different from the source impedance that would result in maximum power delivery to the load. For example, in the context of radio-frequency (RF) generators, the source impedance is often very different from the complex conjugate of the load impedance. In terms of a Smith chart (reflection coefficient chart, Philip H. Smith, 1939), the source impedance in such generators is toward the edge of a chart normalized to the load impedance (e.g., 50 ohms for standard RF applications). Some radio-frequency (RF) generators are designed with such a source impedance to render the generator less expensive and bulky than one having a resistive source impedance (e.g., 50 ohms).
One disadvantage of such a design, however, is that the generator is much more sensitive to variations in load impedance when the load impedance is close to the nominal load impedance (e.g., 50 ohms) into which the generator is designed to operate than a generator having a resistive source impedance that is matched to the load impedance. A particular difficulty in such systems when operated into a nonlinear load such as a plasma is that a change in generator output power can result in a change in load impedance, and a change in load impedance can result in a change in generator output power. In some situations, the generator and the nonlinear load may interact in a manner that results in instability of the output power.
It is thus apparent that there is a need in the art for an improved method and apparatus for modifying interactions between an electrical generator and a nonlinear load.
Illustrative embodiments of the present invention that are shown in the drawings are summarized below. These and other embodiments are more fully described in the Detailed Description section. It is to be understood, however, that there is no intention to limit the invention to the forms described in this Summary of the Invention or in the Detailed Description. One skilled in the art can recognize that there are numerous modifications, equivalents, and alternative constructions that fall within the spirit and scope of the invention as expressed in the claims.
The present invention can provide a method and apparatus for modifying interactions between an electrical generator and a nonlinear load. One illustrative embodiment is a method for modifying interactions between an electrical generator and a nonlinear load connected with an output of the electrical generator, the method comprising receiving a main control signal at a control input of an engine of the electrical generator, the main control signal controlling at least one of output power, output current, and output voltage delivered by the electrical generator to the nonlinear load, the engine being one of a power amplifier and a converter; measuring the impedance of the nonlinear load; and feeding to the electrical generator a compensation signal corresponding to the measured impedance, the compensation signal rendering a transfer function of the output power of the electrical generator with respect to the main control signal substantially insensitive to variations in the impedance of the nonlinear load to stabilize the output power of the electrical generator.
Another illustrative embodiment is an electrical generator, comprising an engine including a control input configured to receive a main control signal, the main control signal controlling at least one of output power, output current, and output voltage delivered by the electrical generator to a nonlinear load connected with an output of the electrical generator, the engine being one of a power amplifier and a converter; and a compensation subsystem including an impedance-measurement circuit that measures the impedance of the nonlinear load and a compensation-signal-generation circuit that feeds to the electrical generator a compensation signal corresponding to the measured impedance, the compensation signal rendering a transfer function of the output power of the electrical generator with respect to the main control signal substantially insensitive to variations in the impedance of the nonlinear load to stabilize the output power of the electrical generator.
These and other embodiments are described in further detail herein.
Various objects and advantages and a more complete understanding of the present invention are apparent and more readily appreciated by reference to the following Detailed Description and to the appended claims when taken in conjunction with the accompanying Drawings, wherein:
An understanding of various embodiments of the invention is aided by an analysis of how instability in the output power of an electrical generator can occur as a result of interactions between the generator and the impedance of a nonlinear load with which it is connected.
Power amplifier 110 includes a control input 130 that receives a control signal C 135. Control signal 135 is used to control the output power 115 produced by power amplifier 110. Control signal 135 is produced by a main power control loop (not shown in
Assuming that changes in generator output power 115 in response to a change in impedance of the nonlinear load 105 occur instantaneously and, similarly, that changes in the impedance of nonlinear load 105 occur instantaneously when the output power 115 into nonlinear load 105 is changed, the system shown in
Assuming these functions are differentiable and using a Taylor series expansion with only the first derivative they can be linearized around the operating point to obtain
is the inner product of the vectors
The first of these vectors models the sensitivity of the generator 100 to changes in the impedance of nonlinear load 105, and the second vector models the sensitivity of the impedance of nonlinear load 105 to changes in generator power 115.
As long as the above inner product is less than 1, a drop in the gain of the main power control loop of generator 100 can compensate for the increase in gain of the output power 115 with respect to the control signal 135. However, when the above inner product is greater than 1, the sign of the transfer function from the control signal 135 to the output power 115 is reversed, and no modification of the gain of the generator's main power control loop can restore stability. In an unstable condition, generator 100 does not produce a constant output power 115 as desired.
The instability that can result due to interactions between generator 100 and nonlinear load 105 is illustrated in
The resulting output power 115 of generator 100 as a function of time is sketched as graph 175 in
Referring now to
Power amplifier 210 includes control input 230, to which main control signal C 235 is fed. For example, in one embodiment main control signal 235 is a voltage. In general, main control signal 235 is used to control the output power, output voltage, output current, or any combination thereof delivered by generator 200 to nonlinear load 205. Main control signal 235 is produced by a main power control loop (not shown in
Compensation subsystem 240 measures the impedance of nonlinear load 205 and generates a compensation signal K 245 that corresponds to (depends on) the measured load impedance. Compensation signal 245, which is fed to power amplifier 210, renders the transfer function of the output power 215 of generator 200 with respect to main control signal 235 substantially insensitive to variations in the impedance of nonlinear load 205. The result is to stabilize the system by linearizing the output power 215 as a function of the main control signal 235. Compensation signal 245 for a given measured load impedance varies depending on the particular embodiment.
Compensation signal 245 can be determined in advance through a suitable calibration such as the following: First, generator 200 is connected with a test load having an adjustable impedance (e.g., a tuning circuit). The load is set initially to a nominal reference impedance with which generator 200 is designed to operate (e.g., 50 ohms). Second, a desired power set point P0 is input to generator 200, and generator 200 is allowed to settle at output power P0. Third, the main control signal 235 is frozen (fixed) at the current value that produces output power P0 into the reference impedance. Fourth, the load impedance is varied, and the compensation signal 245 required to maintain an output power 215 of P0 with that load impedance is recorded. The fourth step is then repeated for as many values of the load impedance as desired. The entire calibration procedure above is repeated for as many different output-power set points as desired.
In an illustrative embodiment, compensation subsystem 240 is implemented using high-speed digital algorithms in what may be termed by those skilled in the art as the “reflection-coefficient domain.” In one embodiment, for example, compensation subsystem 240 is implemented along with other functions of generator 200 in a field-programmable gate array (FPGA). In other embodiments, compensation subsystem 240 is implemented using a processor that executes firmware or software. In general, the functionality of compensation subsystem 240 can be implemented in hardware, firmware, software, or a combination thereof.
In this illustrative embodiment, impedance-measurement circuit 250 is capable of measuring the impedance of nonlinear load 205 approximately once every microsecond, providing for the cancellation of frequencies associated with instability below approximately 500 kHz. This sampling rate is lower or higher in other embodiments.
In one embodiment, compensation-signal-generation circuit 260 includes a lookup table for each of a plurality of output power levels 215. Each lookup table for a given output power 215 maps each of a set of discrete values of the measured load impedance 255 to a corresponding discrete value of compensation signal 245. In such an embodiment, compensation-signal-generation circuit 260 includes a digital-to-analog (D/A) converter (not shown in
In some embodiments the calibration procedure for a given output power level is performed for only a few points (e.g., four load-impedance values other than the reference impedance that bracket the reference impedance on a Smith chart). For other values of the measured load impedance 255, the compensation signal K can be obtained from those few stored values by interpolation, for example. In some embodiments, slopes (gradients) of compensation signal 245 as a function of the measured load impedance 255 are stored in the lookup tables, and the compensation signal 245 for a specific measured load impedance 255 is interpolated by multiplying the appropriate slope by the difference between the measured load impedance 255 and the reference impedance. Also, in some embodiments, fast numerical algorithms such as successive approximation are used to perform mathematical operations such as division, improving the speed of compensation subsystem 240.
The sum produced by summing circuit 430—the sum of main control signal 425 and compensation signal 420—is thus the control signal at control input 410 that causes power amplifier 405 to produce the desired output power P0 into the measured load impedance 255 for essentially the same main control signal value 425, irrespective of load impedance, thus rendering main control signal 425 insensitive to variations in the impedance of nonlinear load 205. Of course, when the measured load impedance 255 is the reference impedance (point 505 in
Compensation subsystem 720 produces a compensation signal 725 specifically tailored for connection with secondary control input 715. Note that the specific compensation signal 725 as a function of load impedance depends on the design of power amplifier 705. Regardless of the design of power amplifier 705, however, a calibration procedure such as that described above can be performed to determine the compensation signal 725 for each of a set of values of the measured load impedance 255 for a given desired output power P0.
The combination of main control signal 730 and compensation signal 725 causes power amplifier 705 to produce the desired output power P0 in spite of variations of the impedance of nonlinear load 205. In other words, compensation signal 725 renders the transfer function of the output power 215 of generator 700 with respect to main control signal 730 substantially insensitive to variations in the impedance of nonlinear load 205, thereby stabilizing the output power 215 of generator 700 as a function of the main control signal 730.
In some embodiments, the compensation signal effectively nulls the inner product in Equation 1 above. That is, the compensation signal nullifies the sensitivity of the power amplifier to changes in the impedance of the nonlinear load. In other embodiments, additional compensation can be applied to the power amplifier via the compensation signal to render the inner product in Equation 1 other than zero, causing the power amplifier and the impedance of the nonlinear load to interact in a particular desirable manner. In some embodiments, this additional compensation to achieve a desired interaction between the generator and the nonlinear load can be specified by a user of the generator. This additional compensation can provide, for example, additional stability beyond that provided by simply nullifying the sensitivity of the power amplifier to changes in the load impedance.
In conclusion, the present invention provides, among other things, a method and apparatus for modifying interactions between an electrical generator and a nonlinear load. Those skilled in the art can readily recognize that numerous variations and substitutions may be made in the invention, its use, and its configuration to achieve substantially the same results as achieved by the embodiments described herein. Accordingly, there is no intention to limit the invention to the disclosed exemplary forms. Many variations, modifications, and alternative constructions fall within the scope and spirit of the disclosed invention as expressed in the claims.
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|U.S. Classification||322/36, 322/37|
|Cooperative Classification||H02P9/105, H03H7/40|
|European Classification||H03H7/40, H02P9/10D|
|Jul 27, 2009||AS||Assignment|
Owner name: ADVANCED ENERGY INDUSTRIES, COLORADO
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:VANZYL, GIDEON;REEL/FRAME:023011/0552
Effective date: 20070408
|Feb 4, 2015||FPAY||Fee payment|
Year of fee payment: 4